The invention relates to an electronically controlled ignition system. In one such system, the timing of the supply of the primary current (1.sub.pr) flowing through the primary winding of the ignition coil is controlled in accordance with the speed so that the said current only reaches the value (I.sub.prmax) required for ignition shortly before the time of ignition. It is established immediately before the moment of ignition whether the primary current has reached the value required for ignition by deriving a test pulse (U.sub.te) from the duration (t.sub.e) of the primary current at the value (I.sub.prmax) required for ignition, said test pulse being used to disconnect the electronic control unit for a fixed period of time when there is no primary current or insufficient primary current to prevent misfiring. The timing of the supply of the primary current is derived directly from the control signal (U.sub.IN) of the ignition pulse generator and once the period of disconnection has been completed the electronically controlled regulating condition is again implemented continuously and automatically.
An electronically controlled ignition system of this type is described in German Patent Application No. P31 11 856.9. In the previously proposed controlled ignition system, a test pulse having duration t.sub.e is obtained from the duration of the primary current at its value necessary for ignition. This test pulse actuates a switch by means of which a capacitor is discharged, said capacitor being charged when there are no t.sub.e pulses. From this there is the result that the voltage drops across the capacitor when test pulses are lower than the value of a comparison voltage which is fixed and pre-determined and is for example half as high as the maximum voltage possible at the capacitor during the following test pulse. This comparison voltage is compared by a comparator with the capacitor voltage, a signal only occurring at the output of the comparator when the capacitor voltage is higher than the comparison voltage during a particular trigger pulse. This can only be the case when there is no test pulse immediately preceding the respective trigger pulse and therefore sufficient discharge of the capacitor is not ensured. However if there is no test pulse this is the same as misfiring, since in this case the primary current through the ignition coil has not yet reached the value required for ignition. The trigger pulse activating the comparator is obtained from the negative disconnection flank or trailing flank of the control signal when this control signal of the ignition pulse sensor passes from its high level to its low level. For example, a mono-stable trigger stage changes from its stable condition to its quasi-stable condition by an output signal at the comparator, so that during the period when the said mono-stable trigger stage is in its quasi-stable condition, the electronic control unit is disconnected and the timing of the supply of the primary current through the ignition coil is derived directly from the control signal from the ignition pulse generator. If the period of disconnection is at an end then the electronically controlled regulating condition is again implemented continuously and automatically in the manner described in the above mentioned patent application.